Television device



19, 1941- P. w. WILLANS 2,252,746

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f z PICTURE Rev/1547mm Slam/-14 Mus/2 AMPLIFIER GENERATOR Pswcs J'Y/vwnamsmc puss beusmran Patented Aug. 19, 1941 TELEVISION DEVICE Peter William Willans, Hampstead, England, assignor to Electric and Musical Industries Limited, Hayes, Middlesex, England, a company of Great Britain Application April 12, 1934, Serial No. 720,205. Renewed June 6, 1940. In Great Britain April 6 Claims. (Cl. 178-75) The present invention relates to signalling systems, such for example as television or picture transmitting systems, in which signalling is effected with the aid of signals of the kind which may comprise oscillations of any frequency between a predetermined maximum value and zero or at least a lower limiting value which presents difficulties in transmission and reception.

In television systems, for example, an object to be transmitted is usually scanned in contiguous strips and electrical variations are derived from the changes in brightness of the elemental areas of the object. When the object has been completely scanned, the operation is repeated. This complete scanning may take place 24 times per second for example. Since the absolute level of brightness has significance to the eye it is necessary to reproduce in the final picture all changes in brightness of the object, however slowly these changes may occur, if the final result is to be an undistorted representation of the object. It will be evident that in order to convey always the correct impression of absolute brightness it is necessary to transmit and receive frequencies down to and including zero frequency. The use of amplifiers capable of handling direct currents as well as alternating currents covering a wide frequency band is usually inconvenient and costly.

Usually, the amplifiers have a low frequency cut-oil, for example they may be capable of amplifying from about 10 cycles per second to 40,000 or more cycles per second, and below l0 cycles per second the amplification falls rapidly. Such amplifiers are almost always resistancecapacity coupled and include as coupling between stages, a condenser arranged in series between the anode of one valve and the grid of the next valve and a resistance between the grid of the second valve and the common cathode lead. The low cut-off frequency of the amplifier is then determined by the time constant of the condenser and resistance. If the anode circuit of the first mentioned valve contains oscillations having components of frequency lower than the cut-oil frequency of the coupling, only the higher frequency components which are passed by the coupling will affect the grid circuit of the second valve. Once the steady state has been reached, the potential of the grid of the second valve will vary in accordance with the wave form of the higher components about an electrical zero in such a manner that the area of the grid potential-time curve above the electrical zero line is equal to the area below that line. The electrical zero is a potential differing from the cathode potential, by the amount of the grid bias and is wholly independent of the D. C. and low frequency components in the signal, which cannot pass through the coupling.

In some known television systems, the picture scanning is interrupted for a short time between the scanning of successive lines and during this short time the picture signal assumes a value corresponding to complete black or a value even further removed from white than this. The pulse is often used for synchronising purposes and will be called the synchronising pulse.

In scanning, relative motion is usually produced between an image of the object to be transmitted and a scanning aperture arranged in front of a photo-electric cell. Where, for example, the object is a transparency such as a motion picture film, the signal generated in the cell will have its maximum possible amplitude in the white" direction (hereinafter referred to as full white") when the part of the image of the film opposite the scanning aperture is that corresponding to the most transparent part of the film. Similarly with any artificially illuminated object, the value of the maximum possible signal amplitude can be determined.

As an example we may assume that referred to black, full white gives a signal voltage of l0 volts and that the synchronising pulse has an amplitude of +10 volts and lasts for a time equal to one tenth of the time taken to scan one line. The explanation will be made clearer by reference to the illustrative diagrams in Figs. 1 to 5 of the accompanying drawings. If first the picture signal be assumed to be in the form representatlve of a black dot, of length equal to one tenth of a line, upon a white ground, occupying the remainder of the line, such as would be obtained by scanning a strip shown in Fig. 2, the picture signal voltage V representative of this strip, referred to black as zero voltage and plotted against time t as abscissa may be in the form represented in Fig. 3 of two square topped waves a and b of 10 volts amplitude each extending over nine twentieths of the line and between them a return to zero extending over one tenth of the line. In addition there is a synchronising pulse 0 of +10 volts lasting for one tenth of the line scanning time. This signal is passed repeatedly through a device having a low frequency cut-off, such for example as that shown in Fig. 1 comprising a series condenser I and a shunt resistance 2, the signal being applied to input terminals 3 and taken ofl from output terminals 4. When the steady state has been reached, the oscillations in th output of the device will take place, as explained above, about an electrical zero (represented by the dotted line d in Fig. 3) such that areas of the potential wave above and below this zero are equal to one another. In the particular case mentioned the amplitudes about this electrical zero will be found to be about 17 volts for the synchronising pulse and 3 volts for the white background. The electrical zero is then 7 volts negative with respect to complete black.

If the picture signal then has to represent a white dot on a black ground as shown in Fig. 4, it will be found as shown in Fig. that the electrical zero is coincident with the potential marked 0 corresponding to complete black and the amplitudes relative to the electrical zero are :10

volts.

It will therefore be seen that because the D. C. component has been removed, the valves of an amplifier require to be biased in such a way that they can handle not only the apparent maximum amplitude, in the case mentioned 10 volts positive and negative, but also a swing from -3 volts to +17 volts. In the particular example given, therefore, the bias would have to be such that the swing could extend between -10 volts and +17 volts without distortion.

If account is taken of all circumstances, it will be found that the swing to be accommodated is liable to be as high as twice that which would be necessary if the D. C. component were present. The same applies where the missing component is. an alternating current of any frequency below the cut-off frequency of the amplifier.

The result is that, in practice, it has not been possible to take full advantage of the fact that the maximum picture signal amplitude, unlike acoustic signal amplitude for example, is a known quantity and to use the amplifiers handling the picture signals efficiently.

Similarly, in the case of modulation in a carrier wave transmitter, the carrier amplitude in the absence of a picture signal may have to be made double what it would require to be were the modification effected with the D. C. components present.

It is an object of the present invention to overcome or at least greatly reduce the above mentioned difiiculties whilst permitting the use of amplifiers which themselves are incapable of amplifying over a relatively wide band of low frequencies including zero frequency.

According to the present invention there is provided a transmission system for signals of the kind referred to wherein the signal amplitude periodically, but not necessarily regularly, attains a fixed maximum or minimum value, characterised in that the direct and low frequency signal components below a minimum transmission frequency which is less than the minimum frequency of the recurrent maximum or minimum amplitude, are not transmitted through the transmission system but are re-created at a desired point by means of the operation of the recurrent maximum or minimum signal amplitude on a device which supplies the direct and low frequency components with reference to the amplitude of said recurrent maximum or minimum signal amplitude.

The present invention also provides a method of transmitting signals of the kind referred to wherein the signal aniplitude is caused to attain periodically, but not necessarily regularly, a predetermined maximum value in one direction and wherein low frequency signal components are removed from the signal and re-inserted at a desired point, the re-insertion being effected under the influence of the periodic maxima. The upper frequency limit of the removed and reinserted components is lower than the frequency of the periodic maxima or than the minimum frequency of these maxima where they are irregular. The re-insertion is accomplished'by arranging that the maxima either themselves constitute the zero of the signal oscillation or are displaced by a fixed amount from that zero.

In electrical carrier wave signalling the present invention provides a method of transmitting signals of the kind referred to wherein the signal amplitude is given a form in which it attains periodically, but not necessarily regularly, a maximum fixed value in one direction, the signal being used in this form to modulate a carrier wave, so that the fixed value is represented by a fixed carrier amplitude.

The present invention also provides a method of transmitting signals of the kind referred to according to which frequencies between zero and a predetermined intermediate value are removed from the signal and are re-inserted into the signal at a desired point at the transmitter or at the receiver with the aid of means responsive to the peak value of a recurrent maximum signal amplitude in one direction. The re-inserting means are capable of providing, in response to the said peak value, all frequencies from zero to a. limiting value which lies between the frequency of recurrence of the peak value (or the minimum frequency of recurrence where this is irregular) and the said intermediate value. The removal of the lower frequencies is usually accomplished by the inherent properties of the transmission channel, including amplifiers, through which the signals are passed. The re-inserting means should be capable of providing all frequencies below the lowest which the transmission channel is capable of transmitting without appreciable distortion.

Other features of the invention will appear from the following description and the appended claims.

The invention will be described by way of example with reference to Figs. 6 to 13 of the accompanying drawings showing various circuits according to this invention, and Fig. 14 shows schematically a television transmitter according to the invention.

A television transmitter as shown schematically in Fig. 14 comprises means 33 any known or suitable type for generating picture signals and means 39 for generating, between groups of picture signals representative of adjacent lines of an object, a synchronizing pulse in the black direction. The amplitude of the pulse will for convenience be assumed to be +V volts, with reference to complete black as zero, and the maximum picture signal amplitude will be assumed to be -V volt, corresponding to complete white. These signals are generated in two photo-electric cells, one generating the picture signals P and the other the synchronizing pulses. The outputs of the two cells are mixed together in a mixer 40 and are fed to the input of a resistance-capacity coupled amplifier. The nature of the signals existing at various parts of the circuit of Fig. 14 are indicated, the picture signals being represented at p and the synchronising signals at s.

The time constants of the couplings of the rescram sistaiice-capacity coupled amplifier 4| may be made about one hundredth o! a second for example. The rrequency of recurrence of the synchronising pulses a may be 3000 per second. In the output of the amplifier ll the oscillations across the output impedance z will take place about an electrical zero line e, the anode end or this impedance varying in potential from positive to negative with respect to the electrical zero, which maybe assumed to be at a fixed potential relative to earth, in such a way that the average area of the anode potentlal-time curve is zero. If at some instant the conditions are such that the amplitude oi the synchronising pulse is +1 V volts relative to the electrical zero where n is the overall magnification oi the amplifier, then it will be evident that at that instant the electrical zero corresponds to complete vblack and the picture signal is truly represen-- tative oi the brightness of the object. It now the average br ghtness oi the object increases, the amplitude of the synchronising pulse in' creases to say +n (V-i-v) volts. In order now to represent this new brightness accurately the amplitude of the picture signal requires to be increased (in a negative direction) by n 2: volts, or in other words the true zero line should be at +11. volts relative to the electrical zero.

This can be done by arranging that the true zero is made n V volts less positive than the peak of the synchronising pulse. In this way the missing D. 0. component defining the absolute brightness oi! the object can in effect be re-inserted.

It has already been explained that in order to enable modulation of a carrier wave to be carried out efilciently, the D. C. and low frequency components of the signal should be included in the signal used to modulate the carrier. The signal has been deprived of the D. C. and low frequency components by the amplifier and these components may therefore be re-inserted by means of the device 42 one example of which is shown in Fig. 6.

Referring to Fig. 6, the earthed terminal of the amplifier output is connected to the cathode 5 0! a modulator valve 0 and the other terminal 01' the output is connected through a condenser 1 to the grid 01" the valve 8, the grid being connected to the cathode through a grid leak resistance 8. In the anode circuit of the modulator is an impedance 9 associated with an oscillation generator (not shown) in known manner such that the amplitude of the carrier oscillation generated is controlled by the voltage developed across the impedance 9. The time constant of the resistance 8 and condenser I is made shorter than that of the amplifier couplings, and in fact shorter than that of any circuit through which the signals have passed since they contained the low frequency components. Furthermore, the time constant of the resistance 8 and condenser I must be considerably longer than the interval between successive synchronising pulses and it may for example be made about one five hundredth of a second. The operation of the circuit is as follows:

It will first be assumed that no picture signal is present so that the signal appearingacross the amplifier output consists of pulses at a frequency of 3000 per second, each of which tends to make the grid of the modulator valve 8 more positive than the cathode 5. Assuming that at first the grid potential is equal to that 01' the cathode, the first pulse makes the grid positive and causes grid current to flow. The condenser 1 thus becomes charged in such a way as to make the grid more negative than the cathode when the pulse is over. The condenser I commences to discharge through the grid leak 8 but, owing to the relatively long time constant, only a little of the charge has leaked away before the next pulse arrives. This again makes the grid p itive, but less so than before, and some grid current again flows, thus still further increasing the charge on the condenser, This process goes on until the grid potential in the absence of a pulse is such that the peak 01 the pulse just causes grid current to fiow.

It now a picture signal is superimposed upon the signal already considered, it will make the grid more negative and will therefore not cause grid current to flow and will not afiect appreciably the average charge on the condenser I. It will be observed, however, that the voltage of the grid is caused by the signal to vary between that atwhich grid current Just flows and a fixed value which is, in the example considered above, 2 n V volts more negative than this. Thus whatever value the electrical zero may assume relative to complete black, the signals upon the grid of the modulator 8 always cause voltage excursions extending from the point at which grid current Just flows, this point corresponding to the peak 01' the synchronising signal and therefore to a predetermined difference from com-- plete black.

In the arrangement described the synchronising pulses constitute regularly recurring maxima oi predetermined value, namely a predetermined voltage difierence from that representing complete black. Because the time constant of the condenser I and resistance 8, acting as the re-inserting means, is short compared with that of all circuits through which the signal has passed since it contained the low frequency components, this condenser and resistance will determine the positionof the signal datum with substantially no interference from preceding circuits. Also because the time constant of the relnserting means is long compared with the time period 01' the synchronising pulses, the datum determined at the commencement of a train of picture signals representing one line of' the object will not vary greatly during the time elapsing before the arrival of the next pulse. Clearly the arrangement will also operate it the recurring maxima are not regularly spaced in time. In this case the time constant of the reinserting means should be long compared with the maximum interval between recurring maxima. Further the arrangement will serve to insert not only the D. C. component but also alternating components of frequency not exceeding some value which is less than the minimum frequency of recurrence of the maxima and which (because the time constant of the re-insertlng means is shorter than that of preceding circuits) is greater than the cut-off frequency of the transmission channel through which the signals have passed.

The transmitted carrier will thus, for maximum picture signal amplitude, be modulated between fixed amplitude limits, one limit being that corresponding to the modulator grid voltage at which grid current just flows and the other limit being displaced from the first limit by the sum or the amplitude of the synchronis n p e and the maximum picture amplitude, due allowance being of course made for amplification occurring in the amplifier and modulator.

At the receiver the carrier is detected and the picture signals so obtained are amplified in a resistance-capacity coupled amplifier. These signals will as before be deprived of low frequency components. In order to re-insert these components in the signals applied to the reproduclng device, which will in this case be assumed to be a cathode ray tube, a procedure similar to that adopted at the modulator valve may be employed. This is illustrated in Fig. 7. The signals are applied to the grid circuit of a valve In which in this case is the last amplifier valve and not a modulator. Otherwise the circuit arrangement in the grid circuit is the same as before. The anode of the valve i is connected directly to the control grid of the cathode ray tube l I and through a resistance l2 and a source of high tension supply l3 to the cathode of the valve. The positive terminal of the source is connected to a variable tapping point on a potentiometer l4 arranged in parallel with a voltage source IS, a point on this potentiometer being connected to the cathode of the cathode ray tube ll.

The potentiometer tapping point is adjusted so .that, in the absence of a picture signaL when the synchronising pulses have reduced the grid potential of the amplifier valve to a substantially steady value at which grid current just flows, the potential on the control grid of the cathode ray tube relative to the cathode thereof has a value corresponding to black. Thus the grid of the cathode ray tube is held by the pulses at a normal potential corresponding to black and any picture signals which arrive will make this grid more positive by amounts truly representative of the original brightness of the abject.

An alternative arrangement to that shown in Fig. 7, whereby the necessity for the balancing battery I5 is removed, is shown in Fig. 8. The condenser I and resistance 8 in the grid circuit of valve 36, which is shown as a screen grid valve, perform the function already described. The

.anode of the valve is connected through resistance 12 to a source of current of such a nature that the voltage is maintained substantially constant over the operating current range. One

,such arrangement is known as a neon stabiliser comprising an enclosed envelope containing neon at reduced pressure. This discharge device has two electrodes arranged in series with a resistance across a suitable voltage source and connections may be taken from the same or other electrodes to the anode circuit of the valve 36. The screen grid of the valve may be supplied from the same source. Across the source is also connected a preferably variable resistance 31 shunted by a condenser 38 in series with a resistance 39 these resistances constituting a potential divider. The cathode of the tube H is connected to the junction point of the resistances 31 and 39. The value of the resistance 31 is then adjusted in the way above described in connection with the adjustment of the potentiometer ll of Fig. 7.

It will be observed that both in the case of the modulator valve and also that of the cathode ray tube, correct settings are dependent upon a knowledge of the amplification which the signals have undergone. As this may sometimes be inconvenient, the synchronising pulse may be given a peak value corresponding to black instead of a peak value displaced from black in a direction opposite to white as in the arrangements described. In this way all settings will be independent of amplification.

It has already been mentioned that the pulses in the black direction transmitted between the scanning of lines of the object may be used at the receiver for synchronising purposes. For example the pulses can be used to control the generation of currents of saw-tooth wave form which serve to deflect the cathode ray across the screen. The separation of these pulses from the picture signal offers difficulty in known systems because of the wandering zero. Due to this the generation of the saw-tooth currents may be interfered with by picture signals and, if the zero wander be in the opposite direction, the amplitude of the pulses may be insufficient to control the generator. A circuit similar to those already described may be used to overcome this difliculty in the following way:

The signals are fed to an amplifier valve l6 shown in Fig. 9 having a condenser I and resistance 8 arranged in its grid circuit as already described. The phase of the signals is such that synchronising pulses momentarily tend to make the grid more positive. The anode circuit of the valve I6 is resistance-capacity coupled to a sec-'- ond valve I1 serving to reverse the phase of the signals. The output of the second valve is provided with selective means l8 adapted to separate the pulses of line scanning frequency (usually of relatively short duration) from the socalled frame pulses, that is pulses, usually of longer duration than the line pulses, which are transmitted between each complete scanning of the object. The line pulses may then be taken from terminals I9 and the frame pulses from terminals 20. The first valve I6 is of high magnification factor and has such a characteristic that its anode current falls substantially to zero with a fairly low negative voltage on its grid. The operating conditions of the valve l6 are made such that the operative range of grid voltage (that is the range between the point at which grid current just flows and the value at which anode current ceases) is slightly less than the amplitude of the synchronising pulses to be applied to the grid compared with black. As before the synchronising signals cause grid current to flow until the condenser I is charged to such a negative voltage that grid current only just flows on the peaks of these signals. Clearly, since the picture signals all tend to make the grid more negative, they will have no effect upon the current in the anode circuit whilst the amplitude of the synchronising pulses developedin the anode circuit will be constant and dependent upon the operative range of grid voltage. This latter can be adjusted, by adjustment of the voltage in the anode circuit of the valve l6, so that the amplitude of the pulses developed in the anode circuit'is just sufllcient to control the saw-tooth generator with certainty.

In all the above examples it has been arranged that the recurrent maxima were in the form of positive voltages. An alternative circuit according to this invention, wherein a diode rectifier is employed, is shown in Fig. 10. It is arranged that the recurrent maxima are in the form of negative voltages. The circuit will be described as applied to the circuit .of a cathode ray tube at a'receiver. The last valve of the receiver amplifier has its cathode connected to the oathode of the cathode ray tube II and its anode connected through a coupling condenser I to the control grid of the tube H. To this control grid is'connected the cathode of a diode rectifier 2|, the anode of which is connected to a tapping point on a potentiometer 22 and through a condenser 23 to the cathode of the cathode ray tube The potentiometer is arranged in parallel 'with a source of voltage 28 and the positive terminal of the source is connected to the cathode of'the tube II. A grid leak 8 is connected acrossthe terminals of the diode. The coupling condenser I and the resistance 8 of the grid leak are given a time constant. as before, which is long compared with the frequency of the signal maxima and short compared with that of preceding circuits. The potentiometer 22 is adjusted so that the normal bias on the grid of the cathode ray tube II, in the absence of any signal, is more negative than that corresponding to black by an amount equal to the amplitude of the maxima relative to black. The signals are fed to the circuit in such phase that the maxima. constituted by the synchronising pulses for example, tend momentarily to make the control grid more negative.

In operation, first assuming that only the synchronising pulses are arriving and that initially the coupling condenser 1 is uncharged, the first pulse tends to make the grid more negative and current fiows through the diode 2!. When the pulse has ceased, the grid will be more positive than before and the condenser I will commence to discharge through the leak resistance 8. The succeeding pulses act similarly until current just fiows through the diode on the peaks of the pulses. Under these conditions the grid will have a potential corresponding to black and picture signalswill be accurately reproduced. If the average brightness of the object increases, more current will fiow through the rectifier and the average grid potential will become more positive thus increasing the average brightness of the reproduced image.

It should be noted that both in the arrangement Just described and also in those involving the fiow of grid current, there is used as means for re-inserting the absent low frequency components the combination of a condenser and a unidirectionally conducting device.

Other arrangements of circuit are possible based on the same general principle of re-inserting the low frequencies by reference to a limiting peak amplitude. For example, instead of the above described arrangements wherein a low impedance amplifier output feeds a series con-' denser with shunt leak and shunt rectifier, a high impedance amplifier output may be used working into a shunt inductance across which is shunted a rectifier and a low resistance in series. The resistance of the rectifier (when passing current) together with the resistance in series therewithmust give with the inductance a time constant lying between the period of the lowest frequency transmitted by the preceding amplifier and the period of the recurrent maxima. Suppose that the direction of amplifier output current which will pass through the rectifier be called positive, then the recurrent maxima must occur in the negative sense. The device will operate to give a current in the resistance in series with the rectifier which is zero for the recurrent negative maxima. Negative pulses arriving from the amplifier will not pass through the rectifier but will charge" the inductance, that is to say initiate current fiow in the inductance. This inductance discharging" through the rectifier and resistance will supply the necessary currents representing the low frequency components. A circuit capable of operating in this way is shown in Fig. 11 in which the signals are applied across an inductance 28 in such a sense that the maxima' make the upper end of the inductance 28 more negative. A diode 21 in series with a resistance 28 is arranged in parallel with the inductance 28 and the potential differences across resistance 28 are applied to the grid circuit of a valve 25. The output containing the re-inserted D. C. component is taken from across the resistance 29.

The above is given merely by way of example to show how a widely different circuit may be used to accomplish the same result. Many other circuits are possible.

As unidirectionally conducting device in the forms of re-inserting means above described, other forms of rectifier than thermionic rectifiers may be used. For example in some cases a contact type rectifier such as a copper-copper-oxide rectifier may be employed.

The re-insertion may take place as often as desired in a transmission system. It is preferably done immediately before any point where overloading may occur.

In all cases described, at any point where reinsertion has been efiected, the steady output in absence of any signal (including the recurrent maxima) has a value representing that normally due to the recurrent maxima. This is not a necessary condition of operation but is in many respects advantageous.

In re-inserting the D. C. components some losses are inevitable. For example, it is a necessary condition of operation of the various reinserting devices described that some current should fiow through a unidirectionally conducting device and a drop of applied voltage thus occurs. The effect of this is that the whole of the missing D. 0. component is not re-inserted. In some cases this may not be material. Where it is desired to obtain more complete re-insertion means may be provided at some suitable point for favouring the D. C. and low frequency signal components relatively to the remainder of the signal frequency band. One arrangement of this kind is shown in Fig. 12.

Here the signal in which the D. C. component is to be inserted is applied to terminals 30 having a condenser I and a resistance 8 functioning in the manner already described. A diode 3| is connected across the resistance 8 as shown. A tapping point on the resistance 8, which may for example be half way along it, is connected through a resistance 32 to the grid of an amplifier valve 33. Across the grid-cathode circuit of the valve 33 is connected a circuit tending to by-pass alternating components. In this case this circuit comprises a resistance 34 in series with a condenser 35 and the values of these components are so chosen that signal frequencies from about 10 cycles per second upwards are substantially uniformly attenuated whilst frequencies below 10 cycles per second are less and less attenuated until at zero frequency there is no attenuation. The output is taken across the resistance 38.

Assuming for example that owing to the action of the condenser I. the resistance 8 and the diode 3|, the voltage across the resistance 8 has the D. C. component present to the extent of regarding the higher frequency components as present to the extent of A fraction, in this case about one half, of this voltage is applied to the grid of valve 33 through resistance 32. Thus in the absence of the circuit 34, 35, fifty per cent of the voltage at all frequencies across resistance 8 will appear across the grid circuit of valve 33, that is to say 45% of the correct a amount of D. C. component and 50% of the higher frequency A. C. components.

similar to that shown in Fig. 6 except that a circuit comprising for example resistance 34 and condenser 35 is shunted across the output resistance 5. This circuit 34, 35, as before, favours the D. 0. component.

Although the invention has been described as applied to television systems, it is not applicable only to television and picture transmitting systems but may be applied to any system where it is required to transmit signals covering. a range of frequencies extending below the lowest frequency which the transmission channel can handle without distortion.

I claim:

1. In a television receiving system, a cathode ray tube having a cathode and a ray intensity control element analogous to a grid, a grid-leak biased thermionic tube having an output circuit including a resistor, a conductive connection between the resistor and the ray intensity control element, means for impressing composite signals including synchronizing impulses and picture signals upon the input circuit of the thermionic tube, and a conductive connection between the cathode of the thermionic tube and the cathode of the cathode ray device, the constants of the grid-leak biasing elements being such that the said thermionic tube functions to apply abiasing potential to the ray control element proportionally to the amplitude of the synchronizing impulses and also to apply picture signals to the ray control element.

2. The invention set forth in claim 1 adlditionally characterized in that the connection between the cathodes of the several devices includes a controllable source of potential.

3. In a television system of the type wherein an area is scanned to develop picture signals and wherein synchronizing impulses are developed periodically, said impulses having a predetermined fixed amplitude with respect to a picture signal representing black, a receiver comprising a grid-leak biased electric discharge tube having an output circuit, said circuit including a resistor through which the plate current of said tube fiows. a cathode ray tube having a control electrode and a cathode, and direct current connections connecting said control electrode and said cathode across said resistor whereby the bias on said control electrode depends upon the fiow of said plate current, the grid-leak biasing elements of said electric discharge tube having values such that the electric discharge tube functions to transfer picture signals with substantially no distortion.

4. In a television system of the type wherein an area is scanned line by line to develop picture signals and wherein a synchonizlng impulse'is transmitted at the end of each scanning line, said synchronizing impulse having a predetermined fixed amplitude with respect to a picture signal representing black, a receiver comprising an electric discharge tube havinga cathode, a grid and an anode, said tube having an input circuit including a condenser in series with said grid and said cathode and including a resistor connected between said grid and said cathode, means for impressing said picture signals and said synchronizing impulses upon said input circult, an output circuit including a source of po-.- tential and an impedance unit having appreciable resistance connected in series between said anode and said cathode, a cathode ray tube having a control electrode and a cathode, and direct current connections connecting said last-named electrode and cathode across said impedance unit for applying picture signals and synchronizing impulses to said last-named electrode, said input circuit having a time constant such that the condenser loses only a small percentage of its charge between synchronizing impulses.

5. A television receiver for the reception of a composite signal including picture signals and periodically recurring synchronizing signals, the synchronizing signal voltage being of greater amplitude than the picture signal voltage and varying in amplitude with respect to the alternating current axis of the composite signal in accordance with the average illumination of a view being transmitted, said receiver comprising a cathode ray tube having a cathode and a control electrode, an amplifier tube having a cathode, a control grid and a plate, means for impressing said composite signal upon said control grid with the synchronizing signals of positive polarity, means for producing a biasing potential in response to said synchronizing signals being impressed upon said control grid and for applying said biasing potential to said control grid to maintain it at such potential with respect to the cathode of said amplifier tube that said amplifier tube functions as a substantially non-distorting amplifier for such picture signals, a plate circult including a resistor connected between the plate and the, cathode of said amplifier tube, and direct current connections connecting the control electrode and the cathode of said cathode ray tube across said resistor.

6. -In a television system of the type in which picture signals and synchronizing impuses are transmitted as a composite signal having an alternating current axis with said impulses of greater amplitude than said picture signals, a television receiver which includes a cathode ray tube having a fluorescent screen, a cathode ray deflecting device, a cathode and a control electrode, means for applying a deflecting wave to said device in synchronism with the occurrence of said impulses, and means for applying a biasing potential to said electrode which varies slowly in response to changes in the height of said impulses with respect to said alternating current axis for controlling the'average illumination of 'a picture formed on said screen, said last means including a grid-leak biased amplifier tube hav- PETER WILLIAM WILLANS. 

